What is the Haas Effect?

The Haas Effect was discovered by Dr. Helmut Haas back in 1949, and it is somewhat of a psychoacoustic phenomenon. This law, also referred to as the “precedence effect,” states that if a sound follows another sound within 40ms (below the threshold of a human’s echo recognition), those two sounds are perceived as a single sound.

In fact, this effect plays an integral part in how we use sound to determine a spatial location. The spatial location is determined by the first noise that we hear. Because there is a very short delay between the two noises in this case, the first sound becomes the dominant sound and takes precedent no matter where the second sound comes from.

Essentially, we determine a source of a sound based on which sound arrives in our listening position first. The short delay or subsequent reflections and noises only help us to perceive spaciousness and depth, even though these sounds are not heard individually.

A Brief History

Hans Wallach, an early 20th-century experimental psychologist, discovered and described the “precedence effect” in 1949. Both he and Lothar Cremer helped to show that two identical sounds presented one after another in close succession would be perceived as a single sound. Their experiments showed that whenever the lag of the second sound was longer than 40ms, no matter what the sound, it was heard as an echo rather than as a piece of the first sound.

It wasn’t until 1951 that Helmut Haas wrote his doctoral dissertation titled “The Influence of a Single Echo On the Audibility of Speech,” which described the “Haas effect.” His experiments showed that a single coherent sound reflection could affect the perception of speech. He carried out his experiments on the rooftop of a freestanding building to help create anechoic conditions.

This combined with a number of other experiments led Haas to the conclusion that humans naturally localize their sound sources from the direction of the first sound, regardless of the reflections that come from the second.

Using the Haas Effect In the Audio Realm

The Haas effect is one of the coolest audio “tricks” that one could add to their arsenal. Very short delay times create a sense of spaciousness, as we said before, while delays that are longer give us a sense of directionality thanks to distinct repeats. This is much of the reason why we typically rely a ton on panning to add a sense of mix directionality.

While the volume going to each channel is controlled by the pan pots, the timing of each channel is controlled by delay. By capitalizing on the Haas effect, we can use both timing and intensity to perceive sounds instead of relying solely on panning, which can create some pretty incredible outcomes.

Creating a Wide Stereo Image

One of the most popular uses of the Haas effect is to create a wider stereo image. This helps to craft mixes that are broader and more dimensional. You can begin by taking a single mono audio track and duplicating it. Pan the original hard left and the duplicated version hard right (or vice versa). Next, add a delay to one of those tracks.

The results are dependent on the delay time that you set, so make sure to experiment a bit until you find something that sounds good within your mix. Of course, it is important to remember that your goal here is to create a wider sound using mono tracks that are identical.

If you use a delay time that is equal or less than 5ms, you can expect an out-of-phase sound that enhances the directionality. While this may be effective depending on your mixing goals, this again is not what we’re after in this case. If you delay your left channel by 5 ms, for example, the right channel will sound far more intense.

The more delay that you add, the more you will enhance the directionality of your sound, although this only works up to a certain point. Once you begin moving past the 10ms mark, your tracks won’t have skewed intensity in one channel or the other. Instead they will sound much wider.

You want to make sure that you stay below 35-40ms as that is where the echo threshold of our ears lies. In doing so, you’ll make sure that there are no distinct audible repeats. With this method, you can give a flat mono track a sense of width and dimension without adding any reverb or stereo imaging.

Directional Masking

The principle of masking states that if two sounds sit within the same frequency range, the louder of the two sounds will become the dominant one. This can be anything from two similar instruments to an instrument and its reverberation. Because of directional masking, in the past we would perceive reverberation as weaker in monophonic recordings since both the signal and the reverberation of that signal came from the same speaker.

We then entered the age of stereophonic recordings, which gave us the ability to use and perceive more reverberation. This is because we were able to spread the reverberation information away from the direct signal, creating a more intelligible recording overall.

Unfortunately, panning doesn’t always work when we’re trying to get rid of directional masking in our mix. This is one reason why we use the timing components in the Haas effect as a hearing trick to help us cleverly move about the stereo field. Use a stereo delay and set one of the channels to 0ms while tweaking the other between 5ms and 40ms until it sounds right.

Use in Reverb Plugins

By utilizing the early reflections that are present in most reverb plugins, we have the ability to thicken up our mix elements. Early reflections typically occur far below the 40ms threshold, meaning they are within the Haas effect range.

Typically, reverb pushes elements far back in the abyss of a mix as a way to create space and depth. If you only use early reflections and get rid of the reverb tail, you can create a bigger and more present sound that has thicker initial transients.

Is the Haas Effect Mono-Compatible?

One of the most common questions that people ask when they get familiar with the Haas effect is:

“Is this effect mono-compatible?”

The short answer here is no. The Haas effect can sound incredibly impressive when you hear it in stereo, although the tone and level will likely change when mixed to mono—if the sounds don’t disappear completely due to phase cancellation. Of course, you could increase the delay outside of the 5ms-35ms range. However, that would defeat the purpose of utilizing the effect in the first place, as you want to hear a single sound and not discrete echoes.

To combat this, we highly recommend not using the Haas effect on elements of the mix that are very critical. Phones utilize mono speakers, and many FM radios and modern DAB radios convert weaker signals to mono. If you use the Haas effect on a sound that is critical to your mix, it could end up sounding strange when converted to mono.

You might try to apply the Haas effect to electric guitar doubles or extra backing vocal layers so that they sound thick and wide when you are listening in stereo. In a mono mix that is fairly crowded, you won’t miss these layers as much since you’ll still be able to hear the original parts.

Mixing with the Haas Effect

The Haas effect isn’t as crazy as many people make it out to be. Now that you are a bit more familiar with how it works and what mix engineers use it for, it is time to use this technique in your own mixes!

There are a wide variety of ways that you can make use of this psychoacoustic phenomenon in your mixes to help create results that are distinctive. Whether you are looking to reduce directional masking in your mixes, enhance the overall stereo imaging of your mixes, or thicken up your mix transients with early reflections on your reverb plugin, the Haas effect can get the job done.